This application describes steering actuators that are also described in a previously filed co-pending application entitled “Steering Actuator for a Skid Steer Vehicle” (U.S. patent application Ser. No. 10/869,274) that is invented by the same inventor and is assigned to the same assignee as the present application.
This application and the '274 application both disclose a steering actuator arrangement. This application also discloses a control system that that is not disclosed in the '274 application and to which the claims of the present application are generally directed.
Skid steer vehicles such as skid steer loaders are a mainstay of construction work. In their most common configuration, they have two drive wheels on each side of a chassis that are driven in rotation by one or more hydraulic motors coupled to the wheels on one side and another one or more hydraulic motors coupled to the wheels on the other side.
The wheels on one side of the vehicle can be driven independently of the wheels on the other side of the vehicle. This permits the wheels on opposing sides of the vehicle to be rotated at different speeds, in opposite directions, or both. By rotating in opposite directions, the skid steer can rotate in place about a vertical axis that extends through the vehicle itself.
The vehicles have an overall size of about 4×8′ to 7×12′ feet which, when combined with their ability to rotate in place, gives them considerable mobility at a worksite. This mobility makes them a preferred vehicle.
Skid steer vehicles commonly have at least one loader lift arm that is pivotally coupled to the chassis of the vehicle to raise and lower at the operator's command. This arm typically has a bucket, blade, or other implement attached to the end of the arm that is lifted and lowered thereby. Perhaps most commonly, a bucket is attached to the arm and the skid steer vehicle. This bucket is commonly used to carry supplies or particulate matter such as gravel, sand, or dirt around a worksite.
Skid steering provides its own problems. First, skidding tears the terrain over which the vehicle travels. The tighter the vehicle skid steers, the more damage to the ground it causes. A skid steer vehicle that turns in place can actually dig ruts into loose soil. Skid steer vehicles operated in close quarters rapidly denude soil by literally scrubbing the ground clean of grass and other plant matter.
The problems with skid steering have limited the use of skid steer vehicles to construction sites, road construction sites and other locations where there is no topsoil or plant matter to damage.
To answer the need for less ground-damaging vehicles, skid steer vehicles have been devised that provide limited steering of the drive wheels with respect to the chassis.
These skid steer vehicles have four wheels pivotable with respect to the chassis by four hydraulic cylinders. The cylinders are hydraulically coupled such that a single hydraulic valve is capable of simultaneously steering all four suspensions at once. When hydraulic fluid flows in a first direction, the front wheels are steered to the left and the rear wheels are steered to the right. When hydraulic fluid flows in the opposite direction, the front wheels are steered to the right and the rear wheels are steered to the left.
There are some limitations to this arrangement, however.
First, skid steer vehicles are controlled in a manner incompatible with wheel steering. It is difficult to adapt skid steer controls to a vehicle that wheel steers.
Traditional skid steer vehicles typically use twin levers located on either side of the vehicle to both move and steer. The lever on one side controls the speed and direction of the wheels on the left side of the vehicle. The lever on the right side of the vehicle controls the speed and direction of the wheels on the right side of the vehicle. Push the lever on one side of the vehicle forward, and the wheels on that side of the vehicle rotate forward, pull the lever back, and the wheels rotate backward. The farther you push (or pull) the lever, the faster the wheels turn. Thus, the two levers simultaneously control speed, direction of turning, and direction of rotation.
Manufacturers have recently developed skid steer vehicles that use a single joystick for all three functions. The operator pushed the joystick straight ahead to go straight forward. The vehicle controller responsively drives all four wheels forward at the same speed. The speed is proportional to how far forward the driver pushes the joystick. To go straight backwards, the operator pulls the joystick backwards. Speed is proportional to how far back the joystick is pulled.
To turn toward the right or left while traveling forward or backward, the operator pushes (forward travel) or pulls (backward travel) the joystick, while also pulling it to the left or right.
To skid steer (in place) to the left, the operator tilts the joystick to the left but does not push or pull on the joystick. To skid steer (in place) to the right, the operator tilts the joystick to the right but does not push or pull on the joystick.
Thus, the newer joystick-equipped vehicle use a single operator input device for steering angle (left/right), direction (forward/backward) and vehicle speed.
In automobiles and trucks with wheel steering, however, the operator selects a desired steering angle with one device: a steering wheel. He selects the vehicle speed with another device: a gas pedal. He selects the vehicle direction of travel with a third device: a shift lever. Thus, each parameter of travel—steering angle, direction and speed—is controlled by its own separate operator input device.
Incorporating wheel steering into a lever or joystick system is difficult. As a practical matter, a steering wheel cannot be used in addition to or in place of the two levers or the joystick. The wheels must be steered using the lever and joystick commands.
Second, it is difficult to change a skid steer vehicle over from skid steering to wheel steering without changing the way the vehicle responds to the operator's steering commands.
After hours, days, months and years of use, a vehicle operator learns to anticipate the movement of a vehicle in response to his operation of the controls. He drives a vehicle almost automatically, anticipating its every response before it occurs. Operating the vehicle, in that sense, is much like playing a musical instrument. Any change in the way a vehicle responds to its controls is jarring at the least. If it is too different, it may reduce the driver's productivity to that of a beginner. Imagine, for example, a car with reversed steering: when you turn the steering wheel to the left the car turns right right, and vice versa. It could easily take months to unlearn one's automatic reactions.
Adding wheel steering to a skid steer vehicle poses the same problem. When the driver moves the levers a certain distance he expects the vehicle to respond identically. If it does not, he may get confused, become unsure of himself, and resist operating the vehicle, depending upon the degree the vehicle's response has changed. Adding wheel steering to a skid steered vehicle requires the careful tailoring of steering response. This is not a trivial task.
Third, it is difficult to change over automatically from skid steering to wheel steering in the same vehicle without radically changing the response. Under ideal circumstance, wheel steering should completely substitute for skid steering, and that wheel steering should “feel” the same to the driver as the skid steering “feels”. The response should be identical.
There are times, however, when the system must change from one mode to another. When the operator is driving rapidly along flat ground in one direction, in theory it should be relatively easy to either steer the vehicle by skid steering or by wheel steering. However, it is unclear how the vehicle should respond when the operator brings it to a halt and then commands it to turn in place. Turning in place or any other form of turning that is too sharp for the wheel steering alone to accommodate requires some skid steering in addition to wheel steering.
Designers of skid steer vehicles that provide both skid steering and wheel steering face these problems. Designers of vehicles that steer both by skidding and by wheel steering and use dual levers or joysticks that separately and independently drive wheels on opposite sides of the vehicle face these problems.
An object of this invention is to provide a smooth transition from a wheel steering to a skid steering mode of operation for these types of vehicles. It is also an object of this invention to provide a steering system for a skid steer vehicle that provides the same response in a wheel steering mode as it does in skid steering mode. It is also an object of this invention to provide a control system for a skid steer vehicle that receives commands from a two lever or joystick arrangement and translates them into wheel steering commands